1. Field of the Invention
This invention relates to improvements in ladder constructions. More particularly, it concerns ladders made of metal channel members structured so that there can be interlocking of side rail and cross member elements of the ladders without use of rivets or other separate fasteners and/or welding to provide the interlocking.
2. Description of the Prior Art
Wood has been historically a basic construction material for all types of ladders. Important features desired in all ladders are (1) lightness in weight, (2) great strength, (3) good stability during use and (4) highest safety possible for the ladder user. Use of wood as the construction material seriously limits the maximizing of this combination of features. The development of technology of light metals, particularly magnesium and aluminum alloys, has promoted their use as construction materials for ladders to such extent that wood has been replaced as the main construction material.
Prior to World War II, stepladders were primarily made from wood which allowed, because of its inherent lightness volumetrically, the use of thick sections which were in turn necessary because of the low volumetric strength and perishability of wood as a structural material. As wood was a relatively inexpensive material in the forms used for stepladders, the thick sections did not produce undue burden from a total expense standpoint and greatly contributed to the rigidity as well as joint possibilities in this construction. In order to achieve the desirable joints, the steps were frequently dove-tailed into the siderail and reinforced with steel cross rods or in some cases cleats at the ends of the steps and sometimes in the centers of the steps themselves. These components could then be nailed together securely, or in the case of reinforcing rods, tightened through threaded sections at the ends. Wood, of course, had the disadvantage of low volumetric strength, higher weight for comparable ladders, as well as suseptibility of rot, decay, fire and warpage. There were some aluminum stepladders manufactured in the 1930's and during World War II for industrial and commercial use with sufficiently heavy sections so that torsional and lateral stability could be obtained and corresponding abuse resistance or durability insured. Wood, as well as these heavy duty aluminum stepladders achieved reasonable torsional and lateral stability which is very desirable in stepladder construction.
Subsequent to World War II, magnesium and aluminum stepladders became available for the household consumer and a wide variety of trade and other applcations. Magnesium was an excellent material because of its good strength and light volumetric weight making possible the utilization of heavy sections for strength as well the torsional stability. The preferred construction of both aluminum and magnesium stepladders was the use of rivets for joining siderails to front steps and rear siderails to cross braces. Resistance welding could have been utilized in the case of aluminum without appreciable loss of temper or strength, but its reliability is frequently questionable if not done under the right control conditions. The spot welds themselves are subject, under ladder service, to fatigue due to deflection of the components resulting in fatigue failure. Fusion welding could and was used in some cases for magnesium stepladders and was satisfactory, but all welding was too expensive in relation to riveted assemblies. Other rivet-type joints were achieved through the use of lugs in the step extrusions which extended through the siderail and were completed through rivet-like forming operations.
The riveted assembly of stepladders was not suseptible to automation and mechanization to the degree achieved in extension ladders and consequently, riveted stepladders retain a higher cost per pound or per foot of ladder. Riveted stepladders also, due to the light sections required for cost practicability, were easy to twist in torsion as well to deflect laterally under load. Furthermore, the joint itself all too frequently was made with small sized rivets for lower cost resulting in inadequate abuse and wear resistance, which in turn resulted in a lose or flimsey ladder after a short period of use. Furthermore, the use of riveted joints results in a human element in assembly which, if the operation is not properly performed, contributes to early ladder failure. Sometimes grommet type or hollow end rivets are used giving only minimum pressure and a poor joint at the interface between the ladder components and a very poor ladder serviceability. In many cases, the rivets themselves are made of a material lower in strength than the ladder components which, if small in size deform, reducing the serviceability and the resistance to torsion or deflection of the ladder through continued service. All too often bearing areas in riveted ladders are too small resulting in egg shaped rivet holes after a short period of service.
Aluminum alloys as used for ladders have a higher compressive yield strength than comparable magnesium alloys as well as higher tensile yield and ultimate tensile strength. They, consequently, can be used in thinner sections and are adaptable to a wider variety of fabrication methods. It so happens that these alloys are quite weldable although welding does soften the temper in the area immediately adjacent to the weld. However, this can be controlled and limited through speed of welding plus adjacent cooling and the use of welds in areas of minimum stress which would, of course, apply in ladder construction as the joint areas are areas of minimum beam or column stress. Resistance welding in aluminum is quite satisfactory and does not appreciably affect the temper if properly done, but requires very close process control. Nothing could be more embarrasing in ladder construction than to have resistance welds which were defective or uncompleted particularly as they cannot be seen from the exterior of the product.
Ladder designers and manufacturers have been continually searching for ways to fabricate ladders from aluminum or its alloys without use of rivets and similar fasteners and/or welding. A relatively early development in aluminum ladder construction employed lugs or studs on the ends of rungs or other cross members that extended through apertures in side rails with the lug or stud ends being pressed or crimped about the side rail apertures to fix the separate parts together (see U.S. Pat. Nos. 3,181,651 and 3,232,378). This basic idea was later used in one modified ladder construction (see U.S. Pat. No. 3,571,909) and in yet another modification with metal that could be welded (see U.S. Pat. No. 3,559,763).
Deforming of tabs, lugs or the like to provide interlocking between ladder parts has been employed in yet other ways. For example, tubular rungs have been joined to channel side rails by crimping portions of the rung ends, or sleeves surrounding the rungs, to the periphery of holes in the side rails (see U.S. Pat. No. 3,500,956; 3,528,525 and 3,638,759). Also, flat steps have been fixed to side rails in metal ladders by deforming flanges on the steps within grooves contained in the side rails (see U.S. Pat. No. 3,970,400).
The manufacture of extension ladders has reached an advanced art through the use of formed and pressed members that has achieved not only a good degree of automation and mechanization, but also stiffness, stability and safety, and good appearance. This has been achieved through the use of round and flat rungs of limited tread width. This has not been achieved in the wide tread extension ladders. The problem in all other cases of ladder construction is one of cost and lack of suseptibility to economical assembly methods.
In spite of the extensive development work and design improvements on metal ladder construction in the prior art, there has existed a need for improvements in the construction of ladders from metal sections without use of rivets or other fasteners and/or welding to provide interlocking between the major metal sections of the ladders. Such improvements, in order to be successful in the market place, would need to produce ladders without loss of strength or stability as compared to prior art ladders while reducing the parts and labor costs of manufacture. Since rivets or other fasteners would be eliminated, improvements in weight reduction would be expected if the other criteria would be met.